Field of the Invention
The present invention relates to a power supply apparatus and an image forming apparatus, and more particularly, to a secondary-side rectification circuit of a switching power supply using a transformer.
Description of the Related Art
Hitherto, electrophotographic image forming apparatus, which use electrophotography to copy an image onto recording paper, have been widely used. An image forming apparatus of this type emits light of a laser or the like onto a photosensitive drum charged uniformly to a high positive or negative electric potential, in a pattern based on image data, to form a latent image with electrostatic charges on the photosensitive drum. A developer such as a toner is filed onto the latent image on the photosensitive drum by electrostatic force to develop the latent image on the photosensitive film. A sheet of recording paper is then laid on the developer after the developing, and electric charges having a polarity opposite to that of the developer are applied from the rear surface of the recording paper. The developer is consequently adsorbed to the front surface of the recording paper by electrostatic force to be transferred to the recording paper. The transferred developer is then fixed onto the recording paper by heating and pressing the recording paper. Electrophotography thus utilizes electrostatic force to move a developer in the respective processes and accordingly requires a power supply configured to generate voltages of varying high levels and varying polarities.
Details of an image forming apparatus are described in Japanese Patent Application Laid-Open No. 2007-206414, for example. This image forming apparatus includes a positive power supply configured to output a positive voltage as a transfer voltage for transferring a negatively charged developer onto recording paper. The image forming apparatus of Japanese Patent Application Laid-Open No. 2007-206414 also includes a negative power supply configured to generate a charging voltage for charging a photosensitive drum, a developing voltage for causing the developer to fly onto the photosensitive drum, and a negative transfer voltage for cleaning a transfer roller. These voltages are high voltages of several hundred V to several thousand V, and a booster circuit using a transformer or the like is needed to generate the high voltages. Accordingly, generating four voltages, i.e., the positive transfer voltage, the negative transfer voltage, the charging voltage, and the developing voltage, normally requires four transformers. In Japanese Patent Application Laid-Open No. 2007-206414, for example, the positive transfer voltage and the negative transfer voltage for the transfer roller, which needs a positive voltage and a negative voltage both, are generated respectively by a dedicated transformer and a shared transformer configured to generate the charging voltage. Three transformers in total thus form a booster circuit, thereby keeping the cost down.
A way to make the number of transformers less than the number of output types as in this example is multiple output transformers, which are a common technology in switching power supplies. A multiple output transformer is a transformer in which one primary winding and a plurality of secondary windings having different numbers of turns are wound around a single core, and is capable of generating a different voltage from each secondary winding. Through connection of a rectification element to one of the secondary windings of the multiple output transformer in one direction and connection of another rectification element to another of the secondary windings in the opposite direction, voltages of different polarities can be generated with a single transformer, which helps to generate many voltages of different polarities and different voltages with a number of transformers that are fewer than the number of voltage types.
However, to output a high voltage, in particular, the multiple output transformer needs to be high in the ratio of the number of turns of the primary winding and the number of turns of each secondary winding. This makes the total number of turns of the plurality of secondary windings high and the size of the transformer accordingly large. In addition, setting the ratio of the numbers of turns based on output voltages makes the multiple output transformer a dedicated transformer that cannot be adapted to changes in output specifications and is low in versatility. Further, fluctuations in the output current of one secondary winding affect the output voltage of another secondary wiring, thereby dropping precision and making it difficult to control. The positive transfer voltage, the negative transfer voltage, the charging voltage, and the developing voltage in the example of the related art also need to be changed and turned on/off independently from one another. It is therefore impractical to use a multiple output transformer that cannot be controlled flexibly as a power supply of a type of image forming apparatus that is described in the example of the related art. Although successful in reducing the number of transformers by integrating some of transformers configured to generate the different voltages, the example of the related art still needs a positive voltage generating transformer to generate a positive voltage and a negative voltage generating transformer to generate a negative voltage. In the case where one pair of a positive voltage and a negative voltage is necessary, for example, at least two transformers are therefore needed in the example of the related art.